The present invention relates in general to the field of electric current computed tomography or ECCT, and, in particular, to a new and useful method and apparatus for determining which pattern of current to apply to the surface of a body in order to best distinguish an unknown conductivity distribution within the body from a known conductivity distribution.
Various interior regions and organs of the human body are known to have different conductivity. Knowledge concerning the patterns of this conductivity can be used as a clinical tool. Differences in conductivity can indicate, for example, the presence of breast tumors.
U.S. Pat. No. 4,539,640 to Fry et al discloses a system and method of impedance imaging which utilizes an array of electrodes that are applied to the outer surface of a torso to be examined in a non-invasive manner. Currents are applied to the electrodes and resulting voltages are measured. Calculations are then made to construct an image for impedances in the body. This reference, however, does not teach how a best possible distribution of currents can be applied to the electrodes for best distinguishing the electrical properties of different areas in the body.
An apparatus and method for detecting tumors in living tissue is disclosed in U.S. Pat. No. 4,291,708 to Frei et al, which determine dielectric constants of various local regions in the tissue. This apparatus and method is drawn primarily to detecting tumors in breast tissue. It does not teach how one should go about selecting a current pattern for application to a plurality of electrodes for determining the electrical properties of the human tissue.
U.S. Pat. No. 4,486,835 to Bai et al teaches an ECCT technique, which utilizes an array of electrodes to be applied to an area of the human body and supplied with electric voltages. Currents produced by these voltages are then measured to calculate the electrical properties at a plurality of locations in the body for the purpose of producing a visual representation of the electrical properties.
So-called impedance cameras and other techniques for imaging patterns of electrical properties within the human body and other objects are disclosed in:
(1) R. J. Lytle and K. A. Dines, "An impedance camera: a system for determining the spatial variation of electrical conductivity," Lawrence Livermore Laboratory Report, UCRL-52413, 1978. PA0 (2) R. P. Henderson and J. G. Webster, "An impedance camera for spatially specific measurements of the thorax," IEEE Trans. Biomed, Eng. Vol. BME-25, No. 3, p. 250-254, May 1978. PA0 (3) R. H. Bates, G. C. McKinnon, and A. D. Seager, "A limitation on systems for imaging electrical conductivity distributions," IEEE Trans. Biomed. Eng., Vol. BME-27, p. 418-420, July 1980. PA0 (4) T. Muari and Y. Kagawa, "Electrical impedance computed tomography based on a finite element model," IEEE Trans. Biomed Eng. Vol. BME-32, No. 3, p. 177-184, March 1985. PA0 (a) Guessing a current j.sup.o (p) and applying it to the surface of the body; PA0 (b) Measuring and recording the resulting voltage for the unknown conductivity v (p; .sigma., j.sup.o); PA0 (c) Calculating or analytically computing the voltage for the given conductivity, v (p; s, j.sup.o); PA0 (d) Numerically calculating the difference between the measured and computed voltages; EQU y(1)=(v (p;.sigma.,j.sup.o)-v (p;s,j.sup.o)) PA0 (where p is a point in the body) EQU and j.sup.l =y(1)/.parallel.y(1).parallel. PA0 (e) If j.sup.l (p) and j.sup.o (p) differ by less than the precision of the ECCT system, then j.sub.l =j.sup.l (p).